The easiest and most efficient way to get this achievement is probably the use of the Vortex special move (
or
+
) on groups of enemies.
In order to make this crowd-control move more effective, you can upgrade it in the Skills menu (Skills > Warrior > Vortex). Personally, I've upgraded it to level 5, as this special move is useful throughout the entire game.
Furthermore, this move can be very well combined with all "elemental" (fire/ice/toxic/electricity) achievements. Just equip your melee weapon(s) with a certain elemental upgrade, keep making use of the Vortex move, and upon getting an "elemental" achievement, change the upgrades to another elemental power. Simple as that.